The hot water tank in vending machines for beverages was previously made of metal or metal parts with the effect that the lime in the water deposited as flakes on the metal surfaces, whereby a decalcification for 2 to 3 hours was necessary 2 to 5 times annually. A manufacture of hot water tanks of plastics eliminates the frequent decalcification, but is instead encumbered with the problem of lime particles floating in the water and following said water when the hot water leaves the hot water tank, said plastics for instance being polypropylene which is particularly resistant to lime deposits on the surfaces. The lime particles are formed by lime crystals, which in turn result from the process EQU Ca.sup.++ +2HCO.sub.3.sup.- .revreaction.CaCO.sub.3 +CO.sub.2 +H.sub.2 O
The lime crystals deposit as small particles in the water instead of on the inner side of the water tank. These small particles act as germs to larger lime crystals, which at first float in the water, and later on settle. The CaCO.sub.3 -crystals are of a larger density than water and do not settle immediately because the resulting CO.sub.2 deposits on the crystals and thereby keeps said crystals floating until they have reached a predetermined size.
When the water tank is used, a considerable amount of lime crystals is constantly floating in the water. These crystals can both act on the delivery functions inter alia by settling in the closing mechanisms of the valves, and cause inconvenience to the customer as the water delivered, such as a beverage, will contain visible lime crystals.